DESCRIPTION: It is well established that a decline in muscular strength and size occurs with aging. Exercise countermeasures, such as resistance training, have been shown to be effective for muscle hypertrophy and strength gains in older adults. However, the influence of progressive resistance training (PRT) on single muscle fiber contractile function and how this relates to whole muscle function is not well understood. Using a population of healthy older men and women (age 65 to 80 years) we propose to examine the influence of high intensity PRT on whole muscle function, cross-sectional area (CT scans), and single muscle fiber contractile properties of individual slow-and fast-twitch muscle fiber types of the quadriceps. During a 12-week training program, the volunteers will exercise three times per week with three sets of 10 repetitions per set. The intensity of the effort during training will be maintained at 80 percent of the subject's one-repetition maximum. To assess the adaptations associated with this training regimen, the volunteers will be tested for knee extension strength, power, force-velocity relationship, fatigability, and the muscle cross-sectional area (CT). Muscle biopsies will be taken from the quadriceps (m. vastus lateralis) before and after the PRT program for the determination of single muscle fiber function. These individual slow- and fast-twitch muscle fibers will be analyzed for size (micrometers), peak force (Po), maximal shortening velocity (Vo), force-velocity relationship, and power. In addition, individual single muscle fibers will be evaluated for specific muscle proteins (myosin heavy and light chain isoforms). We will test the hypothesis that the increase in whole muscle function with PRT is, for the most part, a direct result of improvements in the contractile properties within the skeletal muscle fibers. Specifically, we hypothesize that single fiber diameter, Po, Vo, and power will increase in both slow- and fast-twitch muscle fiber types following the 12-wk resistance exercise. This study will be the first to examine the effects of resistance exercise and single muscle fiber contractile function in the elderly. This research will provide a better understanding of the relationship between whole muscle and single cell contractile function and how resistance exercise counteracts the decrements in muscle mass and function commonly observed in older men and women.